1. Field
This disclosure relates generally to electronically programmable fuses (eFuse), and more specifically, to a method and circuit for protecting eFuses from accidental programming during power-up and electrostatic discharge (ESD) events.
2. Related Art
eFuses are generally programmed by forcing a large electrical current through them. This high current is intended to break the eFuse structure which results in an electrical open. For conventional eFuse programming, the eFuse structure to be programmed is typically selected by a decoded address which is generated by a control circuit that is powered by a low voltage supply (VDD). An eFuse programming command is created by additional logic circuitry which generates a program enable signal. The program enable signal is converted from the voltage level of the logic supply (VDD) to the voltage level of the programming supply (PVDD) by a level shifter. The program enable signal may be amplified by several driver stages. If an eFuse is selected by the control logic and a program enable signal is present, a current can pass through the selected eFuse for programming. During an ESD event on the programming supply (PVDD), the voltage on this supply can increase to a high level of several volts. Since the supply voltage of the control circuit and the circuitry generating the program enable signal is not actively powered during the ESD event, the outputs of these circuits can be in an undefined state during the ESD event. This may create conditions for unwanted eFuse programming. In addition, during power-up, if PVDD is ramped up before the voltage supply of the control circuit, this can also create conditions for unwanted eFuse programming.
As a result, with currently used eFuses and the associated programming circuitry, there is a risk that the eFuses are being programmed by high voltages and currents that can occur during an undesirable ESD event. There is also a risk that the eFuses can be falsely programmed during a circuit power up, when it is possible that the circuitry for controlling the programming matrix of the eFuses is in an undefined state due to power sequencing issues.
Accordingly, there is a need for an improved method and apparatus for overcoming the problems in the art as discussed above.